1. Field of the Invention
The present invention relates to an anodic oxidation apparatus and an anodic oxidation method which anodize a target substrate to give electrochemical treatment to the substrate and a panel for a display device manufactured by these apparatus and method and, more particularly, to an anodic oxidation apparatus and an anodic oxidation method suitable to perform treatment for a large target substrate and a panel for a display device manufactured by these apparatus and method.
2. Description of the Related Art
Electrochemical anodic oxidation treatment of a target substrate is in use on various scenes. Such anodic oxidation includes treatment in which a polycrystalline silicon layer is made porous. The outline thereof is such that the target substrate having the polycrystalline silicon layer formed on the surface thereof is electrically connected to a positive potential pole of a power supply via a conductor and immersed in a hydrofluoric acid solution dissolved in a solvent (for example, ethyl alcohol). An electrode made of, for example, platinum is immersed in the hydrofluoric acid solution, in other words, in a chemical, and is electrically connected to a negative potential pole of the above-mentioned power supply. Further, the polycrystalline silicon layer on the target substrate immersed in the chemical is irradiated with light by a lamp.
This causes the polycrystalline silicon layer to partly melt in the hydrofluoric acid solution. Pores are formed where the polycrystalline silicon layer has been melted, so that the silicon layer is turned into a porous structure. The light irradiation by the lamp is intended for producing holes necessary for the reaction of the above-mentioned melting and pore formation in the polycrystalline silicon layer. For reference, such reaction in the polycrystalline silicon layer in the anodic oxidation is explained, for example, as follows.Si+2HF+(2−n)e+→SiF2+2H++ne−SiF2+2HF→SiF4+H2 SiF4+2HF→H2SiF6 Here, e+ is a hole and e− is an electron. Therefore, this reaction requires holes as a precondition and is different from simple electrolytic polishing.
The porous silicon thus produced is made suitable as a highly efficient field emission electron source by further forming a silicon oxide layer on a nano-level surface thereof, which is disclosed in, for example, Japanese Patent Laid-open Application No. 2000-164115, Japanese Patent Laid-open Application No. 2000-100316, and so on. The use of such a porous silicon as the field emission electron source has been drawing attention as opening a door to realizing a new flat display device.
In the anodic oxidation treatment as described above, the value of electric current passing from a target substrate to a cathode electrode via a chemical is proportional to the area of the target substrate (the area of a treatment part). This is because the electric current causes the reaction to proceed, and the reaction evenly occurs at every point within the target substrate. Therefore, when the target substrate has a large area directed for a large display device, the current value required for treatment increases prominently. For example, assuming that a treatment current of about 5A is necessary for a target substrate having a size of 200 mm square, it is necessary to pass through a target substrate having a size of 1000 mm square a current of 125A that is 25 times the aforementioned treatment current. Note that the area equal to 1000 mm square is a numerical value naturally conceivable from future trends of the large display device.
A device which passes such a large electric current necessarily requires a current source and so on which are larger, resulting in an expensive device. Further, the area which a light source irradiates with light also increases, and the cathode electrode is increased in shape, also resulting in a cause to push up the cost of the device. This is also reflected on the manufacturing cost of a substrate which will be manufactured by this device.
Further, different viewpoints reveal such an aspect that the increase in area for the light source to irradiate with light makes it difficult to irradiate the target substrate with a uniform light amount and the larger cathode electrode makes it difficult to secure uniformity in electric field formed between the cathode electrode and the target substrate, thereby deteriorating the uniformity in the anodic oxidation within the target substrate. This is a problem in terms of ensuring of the quality of the substrate to be manufactured.